A CT scanner has included a detection system with an array of photodiode based detector pixels and processing electronics for each detector pixel with an analog-to-digital (A/D). The A/D converter has been employed as a current-to-frequency (I/F) converter that generates a train of pulses with a pulse frequency indicative of an input of photons incident on a detector pixel. Examples of such a converter are described in U.S. Pat. No. 6,671,345 B2 Vrettos et al., filed Nov. 7, 2001, and entitled “Data Acquisition for Computed Tomography,” which is incorporated herein by reference in its entirety, and “A New 2D-Tiled Detector for Multislice CT,” Luhta et al., Medical Imaging 2006: Physics of Medical Imaging, Vol. 6142, pp. 275-286 (2006).
FIG. 1 illustrates an example in which processing electronics 102 include an A/D converter 104 employed as an I/F converter. The A/D converter 104 includes an integrator 106 (an amplifier 108 and an integrating capacitor 110 in this example) and a comparator 112. The integrator 106 integrates, each integration period, current “I” 114 produced by a photodiode based detector pixel 116 in response to radiation 118 impinging on a scintillator of a photodiode based detector pixel 116. The comparator 112 compares the output of the integrator 106 with a preset threshold (TH) 120 and generates a pulse only when the output satisfies the threshold 120. A reset switch 122 resets the integrator 106 in response to the generation of a pulse.
In FIG. 1, digital logic 124 controls the reset switch 122, including closing the reset switch 122 to reset the integrator 106 and opening the reset switch 122. The digital logic 124 also processes the output of the comparator 112. In one instance, this includes counting a number of pulses output by the comparator 112 and determining a time from a first pulse of the integration period to a last pulse of the integration period. From this data, the digital logic unit 124 can generate an output signal indicative of the frequency of the pulses (e.g., number of pulses in an integration period/the time between the first and last pulses in the integration period), which is indicative of the current or charge per unit of time of the detected radiation.
The integrator 106 also integrates, each integration period, a bias current supplied to the integrator 106 input by a bias current source 126. The bias current is required to ensure that at least one pulse occur within each integration period (i.e., in the absence of any detected photons and thus a signal from the detector pixel 116) so that a frequency can be determined by the A/D converter 104. However, the current source 120 introduces electronic noise into the input of the A/D converter 104, which may increase the noise floor, relative to a configuration without the current source 120, and, hence, raise the lower limit of dose levels for lower dose scanning applications to dose levels that result in signals that are above the noise floor.
In the current state of the art of CT detectors, phototransistors are not used due to dark current limitations of electronics noise and variations with temperature and accumulated radiation dose.